The present invention relates to self-balancing DC-substitution measuring systems and in particular provides a circuit arrangement which reduces the need for amplifiers having a high common mode rejection ratio (CMRR). Such measuring systems are used in many applications, such as vacuum gauges, gas-composition meters, anemometers, infra-red radiation meters, and radio-frequency circuit power meters.
It has been the practice in the prior art to use negative power-feedback to stabilize the temperature of an electrically heated temperature dependent resistor (TDR). These prior art devices generally make use of a Wheatstone bridge circuit, one arm of which contains the TDR element. A differential amplifier is then used to sense the magnitude and direction of imbalance in the bridge and causes the current through the arm of the bridge containing the TDR to change such that the temperature, and therefore the resistance, of the TDR changes, thereby rebalancing the bridge circuit.
Although measuring systems of this kind are very accurate, they suffer from one common drawback. As the variable being measured by the system alters, the differential amplifier changes the voltage applied to one side of the bridge circuit, which in turn changes the common mode voltage at the input to the amplifier. Therefore, in order to obtain a high degree of accuracy from the measuring system, an amplifier having a high CMRR is required.
Another type of prior art self-balancing DC-substitution measuring system is described in U.S. Pat. No. 4,008,610 by Neil T. Larsen and Gerome R. Reeve. The arrangement devised by Larsen and Reeve overcomes the requirement in the prior art Wheatstone bridge arrangement for an amplifier having a high CMRR. However, the Larsen and Reeve system requires two independently floating bi-polar power supplies which adds considerably to the cost and bulk of the system.